1. Field of the Invention
The present invention relates to a position measuring arrangement for determining the relative position between two objects, including a light source and a signal generator that generates displacement-dependent output signals.
2. Discussion of Related Art
For highly precise position measurements, position measuring arrangements on an optical basis are known, which employ interference effects between various partial beams of rays superimposed on each other for generating displacement-dependent output signals. Lasers, which provide radiation with sufficient coherence properties, are often used as light sources in such position measuring arrangements. The lasers used can be, for example, gas lasers, solid state lasers, but also semiconductor lasers or laser diodes. The former are used in connection with position measuring arrangements embodied as interferometers, the latter in connection with position measuring arrangements which, inter alia, include one or several gratings for generating displacement-dependent output signals. With both laser types a stabilization of the provided wavelength is required, since in case of fluctuations in the respective wavelength erroneous measurements would result in the position determination.
Slow drifting of the provided wavelength results in a change of the measured position, in particular in connection with interferometers, a corresponding effect is less with position measuring arrangements with gratings.
In many measuring situations a slow drift of the respective wavelength plays only a small role, for example when it is necessary to perform only relative measurements in a very short time. However, even with such applications this is critical if rapid changes of the respective wavelength occur. However, exactly this can occur in single-mode lasers of various types, i.e. also with gas, solid state or semiconductor lasers. The reason for this lies in the different dependencies of the amplification profile and the resonator modes on the temperature and other influence quantities. Once the maximum of the amplification profile has been displaced too far from a resonator mode, the wavelength of the laser jumps to the adjoining resonator mode. In actual use, the temperature is substantially decisive for this.
A first possibility for solving this problem regards performing a wavelength stabilization of the respective laser, such as disclosed in U.S. Pat. No. 5,161,165, the entire contents of which are incorporated herein by reference. It is proposed in this publication to specifically suppress the multi-mode operation of a laser diode with the aid of an external etalon. However, this way of proceeding requires a relatively large outlay.
Alternatively, it is known from U.S. Pat. No. 5,198,873, the entire contents of which are incorporated herein by reference, for example, to use in place of a so-called single-mode laser diode a multi-mode laser diode, which has a relatively dense mode spectrum and wherein only insignificant changes in the wavelength result in case of small temperature fluctuations. However, such multi-mode laser diodes do not provide the required radiation output, in particular when the radiation from the light source is to be provided via a fiber-optical waveguide to a scanning head with signal generating means. Such a light source cannot be used in connection with required radiation outputs of 10 mW in the scanning head of the position measuring arrangement.